The present invention relates to a circulation control airfoil and more particularly to an adjustable blowing slot for a circulation control blade for a helicopter.
In a circulation control rotor (CCR) helicopter, the blades are rigidly fixed to the hub and lift is controlled by controlling the circulation of air around each blade. The primary advantage of a CCR helicopter is the reduction of weight of the entire rotor and reduction in number of moving parts associated with the blades; both of these result from the fact that the lift of each blade is modified by changing the circulation of air around it rather than by changing its angle of attack. The resulting rotor is lighter in weight thereby allowing a greater payload for the craft, and also is more reliable.
Circulation control airfoils utilize jets of air blown tangentially to the upper surface of the trailing edge of the airfoil to generate high lift coefficients. Generally the chordwise position of the blowing slot is determined prior to construction so as to inject the jet at the most beneficial chordwise position. The chordwise position is determined by calculating theoretical pressure distributions for the angles of attack and lift coefficients the airfoil is expected to experience. The jet should be located prior to the aft pressure peak so as to maintain airflow attachment on the airfoil. By increasing the circulation about the airfoil, high lift coefficients can be achieved.
Various devices have been employed to make the height of the blowing slot adjustable. One such arrangement is shown in U.S. Pat. No. 3,873,233, which issued Mar. 25, l975, to Drew W. Linck. In this airfoil, two blowing slots are provided and adjustment is made by screws which are locked in a vertical position by a suitable fastener and in the lower skin of the airfoil. In the event the aft blowing slot height needs to be adjusted after the final assembly of the airfoil, the screws provide adjustment for a riding slot blade and a rib structure. The main disadvantage of arrangements such as that shown in the above-referenced patent is that large stress concentrations exist where the screws penetrate the outer skin.